This invention relates to a swivel joint for reverse circulation drills.
Reverse circulation drills which can cope with various geological conditions are now in wide use, handling from a soft ground having an N value less than 50 to a hard layer such as granite. However, a serious problem arises in boring through hard rock such as granite.
FIG. 1 illustrates a reverse circulation drill in operation for forming a bore through a hard rock layer, in which indicated at 1 is a base installed on the surface for mounting a support frame 2 for the drill string. The drill string support frame 2 includes a kelly mast 2a, a hydraulic cylinder 2b having a rod end thereof connected to the lower portion of the kelly mast 2a through a pin joint, and a lift frame 2c supported on the cylinder portion of the hydraulic piston-cylinder through a spherical bearing and adapted to be moved up and down by the inward and outward strokes of the hydraulic cylinder 2b.
A swivel joint 3 is fixed at the center of the lift frame 2c and has kelly bar 22 connected to the lower end of its inner housing. The kelly bar 22 is in engagement with a rotary table 23 of a drilling torque supply machine which is fixedly mounted on the base 1, thereby driving a drill pipe 24, which is connected to the lower end of the kelly bar 22, a crossover sub 25, a drill collar 26 filled with lead, a stabilizer 27 and a roller bit 28. A bent pipe 19 is provided at the upper end of the swivel joint 3 for connection to a suction pump (not shown) through a suction hose 21.
The reverse circulation drill usually employs a roller bit as shown at 28 in a boring operation through hard rock. The roller bit 28 consists of a conical roller cutter with inserts of super hard tips or a combination of a plural number of conical toothed roller cutters 28a of wear resistant material, over which the lead-filled collars 26 are mounted to impose a large drilling load thereon. Therefore, the swivel joint 3 which is located at the top end of the drill string, including the drill pipe and roller bit, is required to support a great load.
Further, in a boring operation through hard rock, the cutters are usually located in asymmetric positions on the bit body in such a manner as to produce an increased crushability by producing an impinging force on the rock. This increases the propelling speeding and improves boring efficiency but subjects the bit body to a localized load, imposing a bending force on the drill pipe which transmits the rotational torque to the bit and as a result flexing the drill pipe to impose a great upward thrust on the swivel joint. Namely, as the roller cutter 28a with inserted super hard tips or a combination of toothed edges on the roller is rotated at the bottom of the pit, the bit body is subjected to an eccentric load which tends to produce a bending moment in the drill string thereby flexing the drill pipe 24 which has a lower rigidity as compared with the drill string portion from the crossover sub 25 to the roller bit 28. The bending moment produced in the drill string varies over a wide range depending upon the conditions of the bottom surface of the pit at which the bit 28 is rotated. Therefore, the amount of downward displacement of the top portion of the drill string also varies over a wide range. The above-mentioned upward thrust can be prevented by moving the lift frame 2c to follow these variations. However, the follow-up is practically impossible since the variations have time durations shorter than the time period which is necessary for detecting a variation and operating the hydraulic cylinder accordingly.
Therefore, in the boring operation, the lift frame is adjusted by the hydraulic piston cylinder in such a position that the load which is imposed on the bit when the drill string is in a flexed state is maintained at a predetermined value. As the operation proceeds, the flexed drill string is intermittently straightened with the top end of the drill string thrust upward and these flexing and straightening motions are repeated during the operation. Every time the top of the drill string is stretched upward, the swivel joint 3 receives a large upward thrust which acts as an irregular impact. Depending upon the nature of the rock and the length of the drill string, the upward thrust sometimes reaches a load several times greater than the total weight of the drill string, so that the swivel joint and drill string support frame are required to be able to hold the drill string securely against the great loads. It follows that the thrust bearing of the swivel joint and the drill string support frame are required to have large capacity and strength, respectively, to cope with the above-mentioned great loads. In addition, the drill string support frame is required to have means for suppressing the mechanical vibrations which are caused by the impinging upward thrusts.